Chambers are defined between the lobes of the helical rotors and the walls of the housing, that move from the inlet side to the outlet side as a result of the rotation of the rotors and thereby become increasingly smaller so that the air trapped in these chambers is compressed.
It is known that oil is injected into the compression chamber of such elements to remove the heat of compression, to lubricate the helical rotors, to prevent corrosion and to ensure a seal between the rotors.
This oil originates from an oil separator where the oil is separated from the outlet air.
It is impossible for all air to be removed from the oil, so that oil is injected that contains a certain amount of air.
This air content can be in the oil in the form of air bubbles or dissolved therein.
As a result there is a risk of cavitation. In an oil flow there are two types of cavitation:                cavitation whereby oil vapour bubbles are formed because the static pressure falls below the vapour pressure of the oil;        cavitation whereby air bubbles are formed in oil flows that contain a certain quantity of air, because a reduction of the static pressure makes the solubility of air in the oil fall.        
Depending on the type of cavitation, damage can occur when the air bubbles or oil vapour bubbles thus formed implode in the vicinity of (metal) components. This damage can be very extensive and can lead to the destruction of the machine.
Such cavitation can occur in an oil-injected vacuum pump element of the screw type under the influence of a fall of the static pressure, more specifically at the outlet of the vacuum pump in the last phase of compression.
In the last phase of compression, the volume of the compression chamber goes to zero, such that the pressure in this chamber can rise above the outlet pressure. As a result, large pressure differences occur between the aforementioned chamber and the inlet, where the pressure can be 0.3 mbar(a) and below.
During the last compression phase, the aforementioned chamber is separated from another compression chamber that connects to the inlet by only one single section of the rotor profiles.
In this section a type of channel forms between the profiles of the rotors or between the rotors and the outlet end face that first converges and then diverges to form a ‘nozzle’.
A leakage flow of gas and oil is possible through this channel from the aforementioned chamber to the inlet due to the large pressure difference between the two, whereby due to the form of the channel and the rotors the speed of this leakage flow becomes so high that the static pressure becomes so low that gas bubbles can form.
Further in the channel the static pressure again increases, such that the bubbles formed implode, such that damage occurs to the rotors and the housing. As a result of this damage the vacuum pump element will no longer function or will do so less well.